Fine materials generated from mining activities are often found well-dispersed in aqueous environments, such as wastewater. The finely dispersed materials may include such solids as various types of clay materials, recoverable materials, fine sand and silt. Separating these materials from the aqueous environment can be difficult, as they tend to retain significant amounts of water, even when separated out, unless special energy-intensive dewatering processes or long-term settling practices are employed.
An example of a high volume water consumption process is the processing of naturally occurring ores. During the processing of such ores, colloidal particles, such as clay and mineral fines, are released into the aqueous phase often due to the introduction of mechanical shear associated with the hydrocarbon-extraction process. In addition to mechanical shear, alkali water is sometimes added during extraction, creating an environment more suitable for colloidal suspensions. A common method for disposal of the resulting “tailing” solutions, which contain fine colloidal suspensions of clay and minerals, water, sodium hydroxide and small amounts of remaining hydrocarbon, is to store them in “tailings ponds.” These ponds take years to settle out the contaminating fines, posing severe environmental challenges. It is desirable to identify a method for treating tailings from mining operations to reduce the existing tailings ponds, and/or to prevent their further expansion.
Certain mining processes use a large volume of water, placing strains on the local water supply. It would be advantageous, therefore, to reuse the water from tailings streams, so that there is less need for fresh water in the beneficiation process. In addition, certain mining processes can create waste streams of large-particle inorganic solids. This residue is typically removed in initial separation phases of processing due to its size, insolubility and ease of sequestering. Disposal or storage of this waste material represents a problem for the mining industry. It would be advantageous to modify this material so that it could be useful in situ, for example as part of a treatment for the mining wastewater.
A number of mining operations yield wastewater streams containing fine particles produced during the processing or beneficiation of ores. As an example, the production of aluminum from bauxite ore according to the commonly-used Bayer process takes place by treating the crushed or ground ore with a hot sodium hydroxide solution to produce alumina (Al2O3), which can be reduced to yield aluminum. The insoluble part of the bauxite ore is carried away as an alkaline aqueous slurry called “red mud.” Red mud is a complex material with characteristics that depend on the bauxite from which it is derived, and on the process parameters that produce it. Common characteristics of red mud include a water suspension of fine particles suspended in a highly alkaline water solution, mainly composed of iron oxides, but having a variety of elements and mineralogical phases. The red mud fluid stream, containing about 7 to 9% solids, is typically sequestered in a containment area (an old excavated mine or a manmade lake called a tailings pond) so that the solids can settle out by gravity. About two tons of red mud is produced per ton of metallic aluminum. The magnitude of red mud associated with aluminum production poses a significant environmental challenge for countries where bauxite is refined. A small country like Jamaica, for example, where bauxite refinement is a leading industry, lacks open land suitable for disposal of the hazardous red mud; moreover, containment problems such as leakage, groundwater seepage and rupture of tailings pond dikes makes disposal of this material even more hazardous.
As another example, iron is produced from an ore called taconite that contains magnetite, an amalgam of iron oxides with about 25 to 30% iron. To extract the iron from the ore, the ore is crushed into fine particles so that the iron can be removed from the non-ferromagnetic material in the ore by a magnetic separator. The iron recovered by the magnetic separator is then processed into “pellets” containing about 65% iron that can be used for industrial purposes like steel-making Ore material not picked up by the magnetic separator is considered waste material, or gangue, and is discarded. Gangue typically includes non-ferrous rocks, low-grade ore, waste material, sand, rock and other impurities that surround the iron in the ore. For every ton of pellets produced, about 2.7 tons of gangue is also produced. The waste is removed from the beneficiation site as a slurry of suspended fine particles, termed tailings. About two-thirds of the tailings are classified as “fine tailings,” composed of extremely fine rock particles more than 90% of which are smaller than 75 microns, or −200 mesh); typically, the fine tailings they have little practical use at the mines, and end up sequestered in containment areas such as tailings ponds.
Another mining operation with similar wastewater handling issues is the production of kaolin. Kaolin (“china clay”) is a white claylike material composed mainly of a hydrated aluminum silicate admixed with other clay minerals. Kaolin, used for a variety of industrial applications, is mined and then processed; dry processes and wet processes are available. Wet processes, used extensively to produce additives for the paper industry, yield a slurry that is fractionated into coarse and fine fractions using a variety of mechanical means like centrifuges, hydrocyclones and hydroseparators. Despite repeated processing, a fraction of the slurry contains fine particulate kaolin that cannot be separated from other fine particulate waste residues. This material is deemed waste, and is sequestered in containment areas, either manmade lagoons or spent kaolin mines.
Trona (trisodium hydrogendicarbonate dihydrate) is a mineral that is mined in the United States as a source of sodium carbonate. After the trona is mined, it is processed by exposing it to aqueous solvents so that the sodium carbonate can be recovered. The insoluble materials in the trona, including oil shales, mudstone and claystone, is carried away as tailings for disposal. Tailings, containing suspended fine particles in a fluid stream, may be transported to confinement areas, like tailings ponds; alternatively, tailings may be pumped into abandoned areas of the mine, with retaining walls or other barriers being constructed as needed to prevent the tailings from entering mine areas that are still active.
Phosphatic ore (fluorapatite) mining is a major worldwide industry, with over 150 million tons of ore mined annually. Domestic mining produces around 30 million tons of ore, about 75% of which comes from Florida. During the extraction of phosphate from the mined ore, a process called beneficiation, significant quantities of waste clay and sand are generated. The approximate ratio of the extracted ore is 1:1:1 of fluorapatite to clay to sand. Thus, with the 30 million tons of ore being mined, around 10 million tons of waste clay and 10 million tons of waste sand must be disposed of annually in the U.S.
The clay that is produced by beneficiation exists in a 3 to 5% (by weight) slurry. The current practice of clay disposal is to store the clay slurry in large ponds known as clay settling areas (CSAs), where the clay is allowed to separate from the water suspension by gravity over long periods of time, for example over several decades. For a typical phosphate mine, up to 60% of the surface area of the mine ends up as CSAs. Estimates are that around 5,000 acres of land is turned into CSAs annually in central Florida. Left untreated it can take several decades before CSAs become stable enough for reuse to be considered. Because of the huge environmental and economic impacts of CSAs, a simple, robust, and cost-effective method for treating the clay slurry waste is needed.
While other methods for separating clay fines from wastewater slurries have been tried for phosphate mining, they have proven to be difficult and costly. For example, the Dewatering Instantaneously with Pulp Recycle (DIPR) process has been under investigation for over 20 years at the Florida Institute of Phosphate Research (FIPR), disclosed in U.S. Pat. No. 5,449,464. According to this disclosure, clay slurry is treated with a flocculant and a pulp material to dewater the slurry. While this approach has been studied for over two decades, its high cost, partly due to capital costs of equipment to dewater the treated slurry to high solids content, has prevented its adoption. There remains a need in the art, therefore, for an effective and economical approach to treating the clay-bearing wastewater slurry that is produced during phosphate beneficiation.
As another example, potash, originally known as wood ash, refers to a collection of potassium salts and other potassium compounds, the most abundant being potassium chloride. Potash accounts for the majority of potassium produced in the world. Approximately 95% of potash produced is used for fertilizers, and the rest in manufacturing soaps, glass, ceramics, chemical dyes, etc. Mining for potash mainly consists of extraction from buried evaporates using underground or solution mining. The tailings streams produced from potash mining are usually slurry mixtures of clay in combination with high levels of sodium chloride and other salts. When released into the environment untreated, the suspensions in these tailings take a long time to settle, creating tailings ponds that can take up to 40 to about 70% of the mine area. During settling time, the mechanical integrity of the sedimentation is low due to high water content and the area is not fit to be used for any purpose. For potash, it is desirable to treat the tailings in order to facilitate sedimentation of clay and salt suspensions and increase water recovery. However, the high salt content of these tailings proves hostile to most conventional flocculants (e.g., anionic polyacrylamides). It has been observed that the salinity of potash tailings is high enough to cause precipitation and other adverse effects to such flocculants. There remains a need in the art, therefore, for technologies specifically addressing the problems associated with potash tailings treatment.
Treatment processes for wastewater in mining industries have been disclosed in U.S. Pat. No. 8,349,188 (U.S. patent application Ser. No. 12/792,181), the teachings of which are incorporated herein by reference. Treatment processes for wastewater in mining industries including oil sands mining have been disclosed in WO 2010/098786 (PCT Application No. PCT/US09/54278), the teachings of which are incorporated herein by reference. Modifications in these systems and methods would advantageously improve their efficacy and efficiency.